To clarify, the oblique case looks the same as the plural case in this sentence, but it does not mean "foot" is plural. To say "my feet" here, you would say "मेरे पैरों".
"Feet" is in the oblique case, so you use "मेरे". To tell if a word should be in the oblique case or not, check if it is connected to the rest of the sentence with a postposition. A test you can do if your native language is English is to replace the object (in this case, "my feet") with "s/he" and "him/her" and see which sounds correct. If "him/her" sounds correct, use the oblique case.
Because of the oblique case. Here you need the oblique case and this for leg singular is मेरे पैर, for legs in plural it would be मेरे पैरों.
Thank you! So "leg" singular would be another possible answer (if it were an option here).
I use the keyboard. I wrote leg and duolingo graded this as correct. I don't know why it is not an option in the word bank.